Apparatus for optically detecting a position of mark

Abstract

A position detecting apparatus for optically observing an opaque mark formed on a transparent substrate to detect a position of the mark with a high degree of accuracy. The apparatus includes a detection optical system for forming a detected image of the mark, an illumination system for supplying an illuminating light to the mark through an objective lens of the detection optical system, projection means arranged to face the detection optical system through the substrate and forming a conjugate position to the mark, and reflecting means which is settable substantially at the conjugate position. The image of the mark can be detected as either a bright image or a dark image under the conditions where there are a sufficient contrast between the mark and the background and a uniform background density. The shape of the mark and the arrangement of patterns around the mark can be determined as desired.

What is claimed is: 
     
       1. An apparatus for optically detecting a position of a mark provided on an object to be detected, said apparatus comprising: light source means for supplying light;   an illumination optical system for directing the light from said light source means to said mark to illuminate said mark;   reflecting means having a predetermined reflectance;   optical means for projecting the light passed around said mark to form a first image of said mark onto said reflecting means;   said mark and said reflecting means being arranged in a conjugate relation with respect to said optical means so as to superpose a second image of said mark produced by the light which is returned as the result of the reflection from said reflecting means in a coincident condition with said mark; and   detecting means for detecting a reflection image of said mark in accordance with first light which is directly reflected from said mark illuminated by said illumination optical system and second light which is reflected from said reflecting means and passed through said optical means and around said mark.   
     
     
       2. An apparatus according to claim 1, wherein said optical means includes variable transmittance means for adjusting a quantity of light passed through said optical means. 
     
     
       3. An apparatus according to claim 2, wherein said variable transmittance means is adapted to adjust the quantity of light passed through said optical means in accordance with a difference in intensity between said first and second light. 
     
     
       4. An apparatus according to claim 1, wherein said reflecting means includes variable reflectance means for adjusting a reflectance of said reflecting means. 
     
     
       5. An apparatus according to claim 4, wherein said variable reflectance means is adapted to adjust the reflectance of said reflecting means in accordance with a difference in light quantity between said first and second light. 
     
     
       6. An apparatus according to claim 1, further comprising moving means for moving said reflecting means relatively to said object, wherein said reflecting means includes a plurality of reflective surfaces which are different in reflectance from each other, and wherein said moving means is responsive to a detection signal from said detecting means to move said reflecting means in order to position one of said plurality of reflective surfaces within a detection visual field of said detecting means. 
     
     
       7. An apparatus according to claim 1, wherein said reflecting means includes a first reflective surface having a predetermined reflectance and a second reflective surface different in reflectance from said first reflective surface, and wherein said reflecting means satisfies the following   (1-R.sub.G).sup.4 ·T.sup.2 ·(R.sub.H -R.sub.L)≧I.sub.D /I.sub.O     where R H  is the reflectance of said first reflective surface, R L  is the reflectance of said second reflective surface, R G  is the reflectance of said object, T is the transmittance of said optical means, I 0  is the intensity on said mark of said light from said illumination optical system, and I D  is the minimum difference in intensity between said first and second light which is detectable by said detecting means.   
     
     
       8. An exposure apparatus for projecting an image of a pattern formed on a mask onto a surface of a wafer through a projection optical system, said apparatus comprising: a mask stage for supporting said mask;   a wafer stage for supporting said wafer;   an illumination optical system including a light source for directing light from said light source to a given mark on said mask to illuminate the given mark;   a reflecting member mounted on said wafer stage;   a detection optical system having a predetermined visual field;   said reflecting member being provided with a first reflective surface region and a second reflective surface region which are different in reflectance and in location from each other;   said first reflective surface region and said second reflective surface region being arranged in a conjugate relation with respect to said given mark; and   said wafer stage being adapted to be movable in order to locate one of said first reflective surface region and said second reflective surface region in said visual field of said detection optical system.   
     
     
       9. An exposure apparatus for projecting an image of a pattern formed on a mask onto a surface of a wafer through a projection optical system, said apparatus comprising: a mask stage for supporting said mask;   a wafer stage for supporting said wafer;   an illumination optical system including a light source for directing light from said light source to a given mark on said mask to illuminate the given mark;   a reflecting member mounted on said wafer stage;   a detection optical system having a predetermined visual field; and   said reflecting member being arranged in a conjugate relation with respect to said mark and adapted to vary reflectance thereof.   
     
     
       10. A method for projecting an image of a pattern formed on a mask onto a surface of a wafer through a projection objective lens, said method comprising the steps of: supplying an illumination light to a given mark on said mask;   introducing said illumination light to a reflecting member located in a position conjugate to said mark through said objective lens;   introducing a first light reflected from said reflecting member and a second light reflected from said mark to a detector; and   aligning said mask to a predetermined position on the basis of a detection output from said detector,   changing reflectance of said reflecting member in accordance with reflectance of said mark.   
     
     
       11. A method according to claim 10, wherein said reflecting member has a first reflective surface region and a second reflective surface region which are different in reflectance and in location from each other, and wherein said step of changing reflectance includes a step of moving said reflecting member so as to locate one of said first reflective surface region and said second reflective surface region in a visual field of said detector. 
     
     
       12. A method according to claim 10, wherein said step of changing reflectance includes providing said reflecting member with a variable reflectance and varying said reflectance of said reflecting member.